Purpose Primary systemic anaplastic large cell lymphoma (ALCL) is divided into two entities according to the expression of anaplastic lymphoma kinase (ALK). We investigated 18 F-fluorodeoxyglucose positron emission tomography ( 18 F-FDG PET) findings in primary systemic ALCL according to ALK expression. Methods Thirty-seven patients who had baseline PET before CHOP (cyclophosphamide, doxorubicin, vincristine and prednisolone)-based chemotherapy were enrolled. Among them, patients who underwent interim and/or post-therapy PET were further investigated for the treatment response and survival analysis. Baseline PET was analyzed visually and semiquantitatively using peakSUV, and interim and post-therapy PETs were visually analyzed. Results All cases were 18 F-FDG-avid on baseline PET. The peakSUV of ALK-positive ALCL (n =16, 18.7±10.5) was higher than that of ALK-negative ALCL (n =21, 10.0±4.9) (P =0.006). In ALK-negative ALCL, complete response (CR) rate in negative-interim PET was higher than positive-interim PET (100 % vs 37.5 %, P =0.02); however, there was no such difference in ALK-positive ALCL (100 % vs 75 %, P =0.19). The 3-year progression-free survival (PFS) was not significantly different between ALK-positive and ALK-negative ALCL (72.7 % vs 47.6 %, P =0.34). In ALK-negative ALCL, negative interim and post-therapy PET patients had better 3-year PFS than positive interim (83.3 % vs 25.0 %, P =0.06) and post-therapy PET patients (70.0 % vs 20.0 %, P =0.04). In contrast, ALK-positive ALCL had no such differences between PFS and PET results. Conclusions On baseline PET, all cases showed 18 F-FDGavidity, and ALK expression was related to higher 18 F-FDG uptake. ALK-positive patients tend to have better PFS than ALK-negative patients. Negative-interim PET was a good indicator of CR, and interim or post-therapy PET was helpful for predicting the prognosis only in the ALK-negative group.